Abstract
Variations in dissolved silicate (DSi) concentration and ratio to dissolved inorganic nitrogen (DSi:DIN ratio) were investigated in the Yahagi River, Aichi Prefecture, Japan over a three-year period (from June 2000 to June 2003). During the observation period, DSi concentration and DSi:DIN ratio at the downstream station ranged from 75 to 254 μM (average: 162 μM) and from 0.74 to 4.3 (average: 2.3), respectively. Regarding the DSi:DIN ratio, there is a possibility of Si depletion in the Yahagi River and Mikawa Bay during the low flow period. DSi:DIN ratio in the main stream was reduced by high DIN loading from the lower tributaries, and its effect was stronger when the main stream flow was low. From November to May, the low water exchange rate of the dam water leads to diatom blooms and results in the reduction in DSi:DIN ratio in the river water. Therefore, it is suggested that the increased nitrogen loading due to land development and lower river flow induced by water resources development, such as dam construction, brought about not only a decrease in the DSi supply to the coastal sea but also a reduction in DSi:DIN ratio.
